Studies in the past grant period indicated that the cerebral vasculature of adult (18-21 week) spontaneously hypertensive rats adapts to hypertension by hepertrophy of the vessel wall, vasoconstriction and a shift in the systemic arterial pressure range for near perfect autoregulation of blood flow from a normal range of 60-150 mmHg to 100-200 mmHg. Microvascular pressures in the vicinity of capillaries and the smallest venules were normal in SHR and there was no evidence of permanent or temporary closure of arterioles. In the proposed study, the protocol outlined will determine if in the earliest stages of hypertension, the SHR has vascular characteristics suitable for hypertensive life or if the vasculature is essentially normal in early life but must progressively adapt as the systemic hypertension develops. Mammalian intestinal villi have three major shapes and vascular architecture patterns which are represented in rats, rabbits and cats. These anatomical characteristics in concert with the transport characteristics of the mucosal epithelia and possible differences in counter-current exchange and multiplication system efficiency could result in very different conditions in tissue PO2 and concentrations of absorbed materials along the villus shaft in various species. The purpose of outlined protocols is to determine if villus tissue conditions are dominated by the transport and oxygen consumption characteristics of the mucosal epithelia or if the counter-current system establishes villus apical to base PO2, concentration or osmotic gradients. In addition, pressures in villus capillaries of rats, rabbits and cats will be measured at rest and during maximum absorptive hyperemia to determine if the normally low villus capillary pressure, 12-16 mmHg, at rest is maintained during hyperemia and thereby assists fluid absorption. By comparing villus tissue chemical conditions and microvascular characteristics in three different species, it will be possible to determine if intestinal villus physiology during absorption is similar in mammalian species or dependent upon the unique anatomical and microvascular characteristics of each species.